A map projection is any method of representing the surface of a sphere or other three-dimensional body on a plane
89
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8answers
16k views
What's the difference between a projection and a datum?
What's the difference between a projection and a datum?
40
votes
8answers
1k views
One dimensional map of the world?
Bit of a strange question but hope this is OK to ask here.
Has anyone heard of a '1-dimensional' projection of the world map - that is mapping all the points on the globe to a single line?
I was ...
35
votes
7answers
2k views
What strategies, criteria, or rules to use for selecting coordinate systems?
What strategies, criteria, or rules do you use for selecting coordinate systems for
(a) storing,
(b) analyzing, and
(c) displaying GIS data?
(I humbly offer my reply to a related question ...
34
votes
5answers
3k views
Why is the 'straight line' path across continent so curved?
This is the result of mapping the straight line path from a point in US to Poland using Distance Measurement Tool.
Also, planes from Asia to US would travel almost over North Pole.
Why is the path ...
33
votes
3answers
8k views
What is the current Web Mercator projection code?
The web mercator projection, popularised by Google Maps, seems to be given a new EPSG code every couple of years.
EPSG:900913 (Google in calculator text) was an unofficial code
EPSG:3785 - is the ...
30
votes
4answers
3k views
Why has Web Mercator (auxiliary sphere) become the web map standard?
I understand what the difference is between the Web Mercator projection and Web Mercator Auxiliary Sphere (WMAS). I also understand that both Google and Esri have adopted this projection as their ...
27
votes
2answers
799 views
What projection appeared in the NYTimes this morning?
Perhaps not as GIS related as some, but definitely about maps.
Can anyone identify the map projection that appeared in the New York Times this morning? It's not one I've seen before, and a quick ...
27
votes
3answers
802 views
What coordinate system should be used to store geography data for celestial coordinates?
I am doing an astronomy project. I want to have the information about our images stored in a spatially enabled database. This, I would think, should be a very easy special case for GIS functions ...
19
votes
9answers
1k views
Discontinuous or Interrupted map projections?
Do any of the current open source GIS softwares support discontinous projection systems? E.g. sinusoidal, dymaxion, butterfly, etc? If not, are there any affordable commercial ones that do (affordable ...
19
votes
3answers
343 views
Why are Data Projections relevant?
Sorry for my ignorance - especially after being in this area for a long time.
Why is it useful to store data in unusual projections and datums? I understand the value of projections as output ...
19
votes
3answers
1k views
What was the rationale for the non-standard UTM zones near Norway?
UTM zones 30V, 31V, 32V, 31X, 33X, 35X, and 37X do not line up with the rest of the UTM zone grid.
Why was this allowed? According to Wikipedia, UTM was developed by the U.S. Army Corps of ...
19
votes
2answers
402 views
What projections should I use to make my own Globe?
Searching for an answer to this question, I found instructions posted by Gulf of Maine Research Institute showing how to create a globe.
Using manual methods ...
What approach would I take to ...
18
votes
8answers
4k views
Best practices for identifying the unknown coordinate system of a Shapefile
I got a Shapefile but it's coordinate system is unknown and there is no .prj file.
How I can identify it now? Are there any tools that could help?
18
votes
1answer
776 views
How to create an accurate Tissot Indicatrix?
A Tissot Indicatrix is useful method for communicating at a glance the kinds of distortion a given projection is prone to (in the figure below, each of the red circles occupies the same area). I've ...
17
votes
7answers
513 views
Spatial reference for dummies?
Can anyone recommend a good reference or tutorial that explains the principles of spatial references, projections, and coordinate systems? We are a small team of programmers working with GIS ...
17
votes
7answers
9k views
How do I convert State Plane coordinates to latitude-longitude?
Given a specific state plane coordinate system (whether NAD27 or NAD83), are there any resources to convert to latitude-longitude?
17
votes
5answers
962 views
Simplifying geometries (generalization)
What are the recommended ways of simplifying geometries? Keeping in mind projections, and simplifying state geometries for example.
I've heard about converting to an 'equi-distant' projection that ...
16
votes
9answers
353 views
Can you suggest some well-written introductory texts about coordinate system projections?
Even after some time working in this GIS field, I still don't have a solid understanding of spatial references, how they are projected, and most importantly, if you still need to worry about the fact ...
15
votes
6answers
9k views
How can I define projections for multiple shape files in ArcMap 10 at once?
I have over 100 shape files that don't have .prj file and thus when I bring them into ArcMap they show the coordinate system as unknown. I know all of the shape files coordinate system is GCS WGS ...
15
votes
5answers
5k views
Map projection in OpenLayers
I want to overlay some data whose projection is WGS-84 on Google map layer in OpenLayers. But I just can't make them in the right place. I did as follows:
map = new OpenLayers.Map('map', {
...
14
votes
2answers
536 views
Which world map projection allows comparing countries by shape and size?
The Mercator projection increases scale along with the latitude. Greenland seems larger than Australia, when in fact it is much smaller. The equal-area projections however stretch shapes away from the ...
14
votes
1answer
763 views
How does one calculate distortion on Equirectangular Projection?
I am trying to calculate distortion so I can distort overlaying text and forms to precisely match an image of an equirectangular projection.
So, how does one calculate the distortion at a given ...
13
votes
6answers
4k views
Shapefile PRJ to PostGIS SRID lookup table?
I was wondering if there is such a thing as an shapefile PRJ to PostGIS SRID lookup table? Something that can translate the most standard shapefile PRJ definitions into the likely SRID.
When using ...
13
votes
1answer
1k views
How to define new custom projections in QGIS?
I'm trying to define a projection in QGIS. I want to add North America Albers Equal Area Conic. From the spatialreference site I copied the proj4 line:
+proj=aea +lat_1=20 +lat_2=60 +lat_0=40 ...
13
votes
1answer
2k views
Calculating Flow Direction and Delineating Basins from Projected vs. Unprojected DEM Data
This is somewhat of a theoretical question stemming from some discussions with colleagues on the topic of implications with delineating basins with projected (e.g., Albers Equal Area) vs. unprojected ...
12
votes
3answers
5k views
Layers having the same coordinate system should overlap, but they don't. What's wrong?
I put 4 different layers into my ArcMap document, but only one of them will show up on the map! I should be seeing the other 3 layers as well. When I do a Zoom to layer, I can see one layer, but I ...
12
votes
4answers
1k views
Does Re-Projecting an ESRI ShapeFile only update the content of the .prj file?
if i wish to change the projection of some shapefiles from .. lets say NAD83 (4269) to WGS84 (4326) ... and by using some 3rd party app .... does this mean the .prj file is only updated .. and not the ...
12
votes
2answers
188 views
What is the approximate error of the Pythagorean Theorem vs. Haversine Formula in measuring distances on a sphere at various scales?
Many people when first trying to calculate distances between two longitude / latitude pairs ask if Pythagorean theorem works as an appropriate distance function.
Most often people answer "no, the ...
12
votes
1answer
345 views
How is a “conformal latitude” used in ellipsoidal stereographic projections?
At my work, I've been doing some maintenance on internal software that claims to implement a Stereographic projection whose formulas were copied from an old system a long time ago.
From what I've ...
11
votes
7answers
303 views
Public reprojection services
Are there any free public services where I can send one point in a coordsystem and it returns the point reprojected in a specified coordinate system?
I will not be doing many requests, for now it is ...
11
votes
3answers
1k views
Do ArcGIS SpatialReference object factory codes correspond with EPSG numbers?
I've got a project where I'm reading in geometry objects from text files to put them in ArcSDE. These geometry objects have an associated EPSG code, and I'm looking for a way to ensure they're in the ...
11
votes
5answers
3k views
How can I measure area from geographic coordinates?
If I have polygons in geographic coordinates (WGS84), how do I measure the total area each takes on the surface of the earth, taking into account the curvature of the earth?
11
votes
3answers
995 views
Official term for latitude longitude coordinate system?
May I know what is the correct specific term for latitude longitude coordinate system?
With "Geographic Coordinate", it seems to be a general term for every other system?
11
votes
2answers
778 views
Calculate distortion outside a UTM zone
One of my colleagues is working with data that is spread across two UTM zone. The majority of the data is in one zone, with a few outliers in another zone. He would like to know what the area ...
11
votes
1answer
101 views
Assign projection to a satellite image
Today I stumbled upon a picture taken from the ISS looking down Europe and it is really beautiful.
So, I started thinking I could add more to the picture. Like world borders, cities, road network, ...
10
votes
2answers
3k views
What's the difference between EPSG:4326 and EPSG:900913?
I am a bit confused on EPSG:4326 and EPSG:900913. I thought that they were both same. Though as I am placing my features over GMaps, which is EPSG:4326, I need to define the srsname of the WFS layer ...
10
votes
6answers
456 views
Where can I find GIS data to test GIS coordinate operations?
I am trying to find some data that can be used to test GIS coordinate operations from one CRS to another CRS. So far all I have found is the Gold Data set and also various example points in the EPSG ...
10
votes
2answers
250 views
How to reproject raster from 0 360 to -180 180 with cutting 180 meridian
I have a geotiff raster image that has a coordinate system with longtitudes from 0 to 360.
The horizontal center of the image is 180 longtitude. See image below:
I want to transform it to EPSG:4326 ...
10
votes
1answer
384 views
Error assessment of calculation done using unprojected vs. projected data
This question builds off of the one with the subject line "Calculating Flow Direction and Delineating Basins from Projected vs. Unprojected Data.":
Calculating Flow Direction and Delineating Basins ...
10
votes
2answers
180 views
How do I determine the projection of a paper map?
If I'm reading a paper map how do I determine what projection was used to draw the map if such information is not explicitly labelled on the document?
Online it is easy to find resources (for ...
9
votes
9answers
4k views
Batch convert lat longs to UTM?
I've got a .csv file that contains a long list of latitude and longitude points. Is there an efficient way to batch convert the list into a projected UTM coordinate system?
Thanks.
9
votes
3answers
4k views
How does ArcGIS compute the distance between two points with a non-equidistant projection?
This is a follow-up question to my previous one, Can you suggest some well-written introductory texts about coordinate system projections?
Let's assume I'm working with the CH1903 map projection, ...
9
votes
4answers
575 views
World map projection with United States in center
I am creating a map showing flight paths originating in the United States and traveling to Asia and Europe. I need a projection for the world that will focus the United States in the center of the ...
9
votes
2answers
347 views
Is there a way to draw a straight line on a map which also represents a straight line on the earth surface?
I'm trying to understand more on projections.
In this example, why must one calculate the curvature of the earth (great circle) if the map is already projected? Is there a way to draw a visibly ...
9
votes
4answers
622 views
How to reproject spatial data using free libraries?
How can I use free libraries to transform spatial data?
For example,
I want to change the projection of a Shapefile within the code of my C# web application. How do I do that?
9
votes
2answers
1k views
API documentation for Gdal/Ogr with C#
I want to assign projection to shapefile in my web C# application, for this i am using Gdal/OGR/OSR C# bindings and add osr_csharp.dll and ogr_csharp.dll references. But i am facing some difficulty ...
9
votes
2answers
644 views
How can I make a polyline wrap around the world?
I am using leaflet maps to create a representation of a round-the-world challenge. I would like to add a polyline which heads east from tokyo and then appears west of south america on the map - but ...
9
votes
1answer
455 views
Waterman butterfly projection in Mapnik
Like the title says, how would someone configure Mapnik to use the Waterman butterfly projection ?
Otherwise, what other tools would be able to render using this projection ?
9
votes
2answers
242 views
How do I perform a 3 parameter geotransformation and projection in Sql Server 2012?
I have a table with latitude longitude (NAD27) columns in it. I compute two other columns, X and Y, representing Web Mercator (WGS84) location.
Currently I'm using a Arcmap to do this, by applying ...
9
votes
1answer
352 views
How to create an Azimuthal Equidistant map for long-distance radio?
I'm trying to produce world or continent scale maps that use the Azimuthal Equidistant projection. I'm a radio amateur, and to an antenna, the whole world looks like spread out on a flat plane, with ...
